Widespread soil bacterium that oxidizes atmospheric methane Alexander T. Tveita,1, Anne Grethe Hestnesa,1, Serina L. Robinsona, Arno Schintlmeisterb, Svetlana N. Dedyshc, Nico Jehmlichd, Martin von Bergend,e, Craig Herboldb, Michael Wagnerb, Andreas Richterf, and Mette M. Svenninga,2 aDepartment of Arctic and Marine Biology, Faculty of Biosciences, Fisheries and Economics, UiT The Arctic University of Norway, 9037 Tromsoe, Norway; bCenter of Microbiology and Environmental Systems Science, Division of Microbial Ecology, University of Vienna, 1090 Vienna, Austria; cWinogradsky Institute of Microbiology, Research Center of Biotechnology of Russian Academy of Sciences, 117312 Moscow, Russia; dDepartment of Molecular Systems Biology, Helmholtz Centre for Environmental Research-UFZ, 04318 Leipzig, Germany; eFaculty of Life Sciences, Institute of Biochemistry, University of Leipzig, 04109 Leipzig, Germany; and fCenter of Microbiology and Environmental Systems Science, Division of Terrestrial Ecosystem Research, University of Vienna, 1090 Vienna, Austria Edited by Mary E. Lidstrom, University of Washington, Seattle, WA, and approved March 7, 2019 (received for review October 22, 2018) The global atmospheric level of methane (CH4), the second most as-yet-uncultured clades within the Alpha- and Gammaproteobacteria important greenhouse gas, is currently increasing by ∼10 million (16–18) which were designated as upland soil clusters α and γ tons per year. Microbial oxidation in unsaturated soils is the only (USCα and USCγ, respectively). Interest in soil atmMOB has known biological process that removes CH4 from the atmosphere, increased significantly since then because they are responsible but so far, bacteria that can grow on atmospheric CH4 have eluded for the only known biological removal of atmospheric CH4 all cultivation efforts. In this study, we have isolated a pure culture (2, 6) and thus play a key role in the atmospheric CH4 budget. of a bacterium, strain MG08 that grows on air at atmospheric Importantly, atmMOB appear to be very sensitive to many types concentrations of CH4 [1.86 parts per million volume (p.p.m.v.)]. of anthropogenic disturbance, including conversion of forests to This organism, named Methylocapsa gorgona, is globally distrib- agriculture, fertilization, acidification, and nitrogen deposition. uted in soils and closely related to uncultured members of the Thus, efforts to manage this biological CH sink could be more α 13 4 upland soil cluster .CH4 oxidation experiments and C-single cell effectiveifweknewmoreaboutthebiologyofthemicroorganisms isotope analyses demonstrated that it oxidizes atmospheric CH4 responsible for it (6). MICROBIOLOGY aerobically and assimilates carbon from both CH and CO . Its es- 4 2 Recent research has revealed that not only members of the timated specific affinity for CH (a0 ) is the highest for any culti- 4 s USCs can oxidize atmospheric CH since this metabolic capability vated methanotroph. However, growth on ambient air was also 4 was also reported for members of the well-known MOB genera confirmed for Methylocapsa acidiphila and Methylocapsa aurea, – “ close relatives with a lower specific affinity for CH , suggesting Methylocystis and Methylosinus (19 23). In addition to the con- 4 ventional,” low-affinity particulate MMO, these bacteria possess a that the ability to utilize atmospheric CH4 for growth is more widespread than previously believed. The closed genome of M. high-affinity form of this enzyme, which is responsible for oxidiz- gorgona MG08 encodes a single particulate methane monooxygenase, ing CH4 at low partial pressures (19). Importantly however, these the serine cycle for assimilation of carbon from CH4 and CO2,and CO2 fixation via the recently postulated reductive glycine pathway. Significance It also fixes dinitrogen and expresses the genes for a high-affinity hydrogenase and carbon monoxide dehydrogenase, suggesting Increasing atmospheric methane concentrations contribute that atmospheric CH4 oxidizers harvest additional energy from oxida- significantly to global warming. The only known biological sink tion of the atmospheric trace gases carbon monoxide (0.2 p.p.m.v.) for atmospheric methane is oxidation by methane oxidizing and hydrogen (0.5 p.p.m.v.). bacteria (MOB). Due to the lack of pure cultures, the physiology and metabolic potential of MOB that oxidize atmospheric methane | USC alpha | trace gases | isolate | filter cultivation methane remains a mystery. Here, we report on isolation and characterization of a MOB that can grow on air and utilizes he atmospheric concentration of methane (CH4), a potent methane at its atmospheric trace concentration as a carbon and Tgreenhouse gas and a precursor for tropospheric ozone pol- energy source. Furthermore, this strain has the potential to lution (1), has increased from 0.720 to over 1.86 p.p.m.v. since utilize five additional atmospheric gases, carbon dioxide, car- the beginning of the industrial era (2–4). The primary sinks of bon monoxide, hydrogen, nitrogen, and oxygen to supply its metabolism. This metabolic versatility might be the key to life atmospheric CH4 are abiotic oxidation by tropospheric hydroxyl radicals and the activity of aerobic methane oxidizing bacteria on air and this discovery is essential for studying the biological (MOB) in unsaturated soils (5–7). MOB belong to a small number methane sink. of bacterial lineages within the Gamma- and Alphaproteobacteria as well as the Verrucomicrobia (8, 9). A defining characteristic of Author contributions: A.T.T., A.G.H., S.L.R., A.S., M.W., A.R., and M.M.S. designed re- search; A.T.T., A.G.H., S.L.R., A.S., N.J., M.v.B., C.H., and M.M.S. performed research; these bacteria is the use of methane monooxygenase (MMO), A.T.T., A.G.H., S.L.R., A.S., S.N.D., N.J., M.v.B., C.H., M.W., A.R., and M.M.S. analyzed data; the unique multicomponent enzyme system catalyzing an initial and A.T.T., S.L.R., S.N.D., C.H., M.W., A.R., and M.M.S. wrote the paper. oxidative attack on CH4 (9, 10). The first MOB was described in The authors declare no conflict of interest. 1906 (11) but it was not until the early 1990s that cultivation- This article is a PNAS Direct Submission. independent indications of the existence of MOB that live on This open access article is distributed under Creative Commons Attribution-NonCommercial- atmospheric CH4 concentrations (atmMOB) were obtained (12– NoDerivatives License 4.0 (CC BY-NC-ND). 15). Interestingly however, calculations indicated that atmo- Data deposition: The sequences reported in this paper have been deposited in the Gen- spheric CH4 does not supply the maintenance energy require- Bank database (accession no. CP024846). ments of cultivated methanotrophs (6), suggesting either that 1A.T.T. and A.G.H. contributed equally to this work. atmMOB are energetically more efficient, have a higher affin- 2To whom correspondence should be addressed. Email: [email protected]. 0 ity or specific affinity (a s)forCH4, or use additional energy This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. sources. Labeling experiments combined with phylogenetic 1073/pnas.1817812116/-/DCSupplemental. marker analyses demonstrated that these atmMOB constituted two www.pnas.org/cgi/doi/10.1073/pnas.1817812116 PNAS Latest Articles | 1of10 Downloaded by guest on September 25, 2021 MOB were not shown to grow at atmospheric CH concentrations 4 A 47 Soil env. seq. (AY662361) and likely rely on storage compounds during periods of low CH4 32 Soil env. seq. (AJ868264) availability that are built up during periodic exposures to high 72 Soil env. seq. (AY662386) concentrations of CH4 (6, 24). 26 Soil env. seq. (AJ868265) Research on USC clades spans nearly two decades. As no Methylocapsa gorgona MG08 Grassland soil env. seq. (FR720169) cultured representatives are available and even their 16S rRNA 17 genes remained unknown until very recently (25), these bacteria 13 64 Soil env. seq. (JQ838704) USC1 MAG Singleton et al., 2018 could be traced in the environment only by using the gene 99 Acidic peatland env. seq. (EF644609) USCα encoding a subunit of the particulate methane monooxygenase Grassland soil env. seq. (FR720266) 84 (pmoA) as a specific marker (16, 18, 24, 26, 27). As revealed in 76 Soil env. seq. (AJ868263) pmoA-based studies, USCs occur exclusively in soils, with USCα 55 Arctic mineral cryosol env. seq. (AJ01563) being found primarily at pH neutral and acidic conditions, while 57 Soil env. seq. (AY662385) USCγ is thriving mostly in pH neutral and alkaline soils (8, 16). Forest soil env. seq. (JN426904) 30 Acidic forest soil env. seq. (FN564652) Notably, both groups have also been observed in arctic upland 84 Soil env. seq. (AF148521) soils (28). During the last years, information on their distribution, 0.10 81 Candidatus Methyloaffinis lahnbergensis 96 81 abundance, and activity was supplemented with first insights into 75 Soil env. fosmid clone (CT005232) their genomic repertoire as obtained from metagenomics (25, 29– Methylocapsa aurea KYGT (CBA11953) 31), but no complete genome sequence of a USC member has yet 68 Methylocapsa acidiphila B2T (CAJ01617) been reported. 36 Methylocapsa palsarum NE2T (KP715290) In this study, we report the isolation of a pure culture of a Methylocystis sp. SC2 PmoA1 (AJ431386) T member of the USCα clade, strain MG08, the basic morpho- 98 Methylosinus trichosporium OB3b (AAA87220) Methylocystis sp. SC2 PmoA2 (AJ431387) logical and physiological traits of this bacterium, and its com-